Hereinbelow, the principle of a unit for separating fluids, such as air gases, is not described in detail, it being a principle widely described in the literature.
All that is indicated here is that this method, for example for separating the constituents of air, is used to produce oxygen, nitrogen and argon (more rarely krypton and xenon):                in gaseous form, or        in liquid form.        
The gases produced (for example oxygen and nitrogen) are generally compressed at different pressure levels and sent to one or more final consumption points. These points may be local or remote. In the case of remote consumption points, the fluids pass through a distribution network (via pipelines for example).
The liquids produced (for example oxygen, nitrogen, argon, krypton or xenon) are, for their part, stored in cryogenic tanks then transported by trucks or wagons to the final consumption points.
The difficulty in managing such a unit stems notably from the fact that all the productions of these fluids are interlinked and the increase in quantity of one or more product leads inevitably:                to the reduction of one or more other products, if it concerns, for example, a consideration of gaseous phase in relation to a liquid phase of one and the same species,        or to their increase, if it concerns, for example, a consideration of one species relative to others (for example oxygen relative to nitrogen, argon, krypton or xenon).        
Furthermore, the means used for the compression or liquefaction, on a production site (which may comprise a plurality of production units), are multiple. It is therefore best to optimize the use of these means in order to both satisfy the production constraints and minimize the energy consumed for the production. In practice, as an indication, 60% of the costs of production of air gases are generally linked to the consumption in particular of electricity.
The choice of the operating parameters (air charge, liquid production, choice of compression means, and other) is generally entrusted to operators (physical individuals). In some cases, automatic systems drive the production unit (or set of units) by using sophisticated control tools which typically handle the management of a transitional state from a first unit operating point A to a second operating point B.
Different types of control tools are known for managing the production load variations of a production unit. These tools generally use predictive control of the type:                with multi-variable predictive control, “MVPC”,        or even, for example, with advanced predictive control, or advanced feed-forward (AFF) control strategy.        
The tools of the first type (MVPC) generally make it possible to achieve a high degree of optimization because they can drive the system at a point close to its limits (property known as “constraint pushing”). This type of technique may have an optimizer, more often than not linear, associated with it.
One the main gaps to be overcome generally, for these types of tool, is an integration of the problem of optimization (which answers the question: “where do we go?”) and the problem of control (which answers the question: “how do we get there?”). This integration may provoke certain useless oscillations through the feedback, in particular in a transitional phase.
Moreover, this type of control presents a difficulty, even an impossibility, in using logic variables (stopping or starting equipment for example).
This type of control takes no account of any notion of time in the production objectives, but simply defines setpoints to be reached as quickly as possible.
Only the constraints that have a linear (or pseudo-linear) relationship with one or more degrees of freedom (defined by control variables) can be managed appropriately. For example, the releasing of a product into the air (due to a difference between consumption and instantaneous production) is difficult to control by this type of system because the gain (in the dynamic sense) is cancelled suddenly when the corresponding valve is closed.
Such a control, even though it is well suited to performing changes of load (management of the production dynamics), is not truly suitable for determining an operating point that optimizes the use of the energy of the site.
There has been proposed an adaptation of this type of optimizer according to a static form that makes it possible, for example, to manage the liquid production of units connected to a network (document U.S. Pat. No. 7,092,893). This optimizer defines objectives, or targets, with a load change controller (MVPC type). Provision is made in particular to use a predefined time interval (more specifically, a fixed period) to send targets to the controller.
However, such an implementation risks provoking oscillations, or even non-optimal results (typically, a static optimization in a method in a non-stationary state). Moreover, the feedback, in this type of configuration, can be taken into account only with difficulty, and therefore with a risk of offset between the model and the real state of the production unit. Furthermore, it would appear that this type of optimization can be executed only with a very slow and fixed rate.
The present invention improves the situation.